1. Field of the Invention
This invention relates to a method of forming a patterned layer on a substrate such as a semi-conductor wafer.
2. Description of the Related Art
One known method of forming patterned metallic layers is to use what is known as a lift off patterning process. In this process a patterned mask, for example a photoresist is formed or laid upon the substrate surface and then a subsequent metallic layer is deposited. The mask is then removed leaving the subsequent layer upon the substrate surface at the opening or recesses which were provided in the mask. A discontinuity of coverage is required at the edges of the patterns to allow the mask to be removed, typically by solvent. Any minor flashing or “wings” that remain can then be removed for example by a CO2 “dry ice” spray. Important characteristics of successful processes are the complete removal of the mask material and the flashings. Preferably the profile of the deposited material should be clean cut and the edges well matched to those defined by the mask. Lift off patterning technology is expected to be used even more widely on other substrates, such as multi-chip modules, as dimensions in those fields shrink to those that are common place on some semiconductor and opto-electronic wafers.
Evaporation is the most wide spread methodology of metal deposition for lift off applications for interconnects formed on wafer substrates. This is because of the vertical deposition of the material from the point source of the typical evaporator achieves favourable lift-off results. Generally, however, sputtering is the preferred deposition technology for depositing in particular metal films, because the films thereof are of substantially better quality. But sputtering tends to be from a broad source, as compared to the point source of the typical evaporator, and is usually carried out at higher pressures and gives rise to higher energy levels in the deposited material. This is particularly true of single wafer sputtering systems of the cluster tool type, e.g., Trikon Technology Inc.'s Sigma model or Applied Material Inc.'s Endura model. These combine to reduce the directionality of the incident material and enable sputtered material to enter re-entrant features within the recesses or openings or to be re-sputtered by the process gas into such re-entrant formations. In the lift off process, this is extremely undesirable, because, for example, it results in substantial wings or flashing and as a result evaporators have a near monopoly in the lift off processing field.
The drawbacks of sputtering can be readily illustrated with reference to FIGS. 1a and 1b. FIG. 1a illustrates the ideal situation after deposition in which a single layer re-entrant photo-resist 1 has been formed upon the upper surface of substrate 2 and sputtered material at 3 has passed through the opening in the resist to form deposit of the material at 3. Other deposited (either sputtered or evaporated) material 4 is caught upon the photoresist and is discontinuous with the useful material at 3. This enables solvent to pass between deposits 3 and 4 to lift off the photo-resist resulting in the well-defined deposit 3, which is illustrated in FIG. 1b. 
In practice, if the deposited material is sputtered, at worst an effectively continuous film forms between 3 and 4, resulting in no solvent penetration and hence no lift off and, at best, significant wings or flashings are formed.